Smartphones and other mobile devices have rapidly become ubiquitous throughout the world. Mobile phones and tablet computers are commonly seen in use at restaurants, in waiting rooms, or on street corners. Mobile devices are used for gaming, photography, listening to music, social networking, or simply talking with another person via a built-in microphone and speaker.
Mobile devices enrich lives by keeping family and friends in communication, allowing any moment to be captured as a photo or video, and providing a means of contacting someone in an emergency situation. FIG. 1a illustrates a mobile device 10. Mobile device 10 is a touchscreen slate cellular (cell) phone. In other embodiments, mobile device 10 is a tablet computer, pager, GPS receiver, smartwatch or other wearable computer, laptop computer, handheld game console, or any other device utilizing capacitive proximity or touch sensing.
Mobile device 10 includes proximity sensor 11. Proximity sensor 11 detects the distance of a user from a front face of mobile device 10. Proximity sensor 11 uses the self-capacitance of a sensing element to determine whether a user is in proximity. Proximity sensor 11 also determines the distance of the user from the front face of mobile device 10. Self-capacitance of the sensing element changes as a user's body part moves nearby proximity sensor 11. The operating system of mobile device 10 is programmed to react when proximity sensor 11 reports a user is in proximity to the mobile device. In one embodiment, radio frequency (RF) output power of mobile device 10 is reduced when a user is in proximity of the mobile device to prevent exceeding specific absorption rate (SAR) regulations. SAR is a measure of the rate at which energy is absorbed by the human body when exposed to an RF electromagnetic field.
Mobile device 10 includes touchscreen 12 on a front side of the mobile device. Touchscreen 12 is used to display a graphical user interface (GUI). The GUI on touchscreen 12 presents feedback, notifications, and other information to a user as determined by an operating system of mobile device 10. Touchscreen 12 is sensitive to physical touch from body parts of a user of mobile device 10. Touchscreen 12 utilizes resistance, capacitance, acoustic waves, an infrared grid, optical imaging, or other methods to determine the presence and location of a user's touch.
In one common usage scenario of mobile device 10, touchscreen 12 displays a button as a part of the GUI, and a user touches the location of the button on the touchscreen to perform an action associated with the button. In one embodiment, touchscreen 12 displays a 3×4 telephone keypad. A user dials a telephone number on the displayed keypad by touching touchscreen 12 at the locations where the desired numbers to dial are displayed. Touchscreen 12 displays an alphanumeric keyboard along with, or as an alternative to, the telephone keypad, with a user touching the touchscreen in the location of letters, numbers, or symbols to be entered in a text input field displayed on the touchscreen. Touchscreen 12 is also used to watch downloaded or streamed videos, or play games, with a user's touch controlling playback of the video or play of the game. In some embodiments, touchscreen 12 is sensitive to a user's touch when the display component of the touchscreen is disabled. While listening to music, a user pauses the music, or advances to the next track of music, by drawing a symbol on touchscreen 12 even though nothing is displayed on the touchscreen.
Buttons 14 provide an alternative user input mechanism to touchscreen 12. Buttons 14 perform functionality depending on the programming of the operating system running on mobile device 10. In one embodiment, buttons 14 return the GUI on touchscreen 12 to a home screen, go back to a previous GUI screen, or open up a menu on the GUI. In other embodiments, the functionality of buttons 14 changes based on a context displayed on touchscreen 12. In one embodiment, buttons 14 are implemented using proximity sensors similar to proximity sensor 11. A user placing a finger on one of buttons 14 modifies the self-capacitance of a sensing element under the button. When proximity is detected, the proximity sensor for the respective button notifies the operating system of mobile device 10 to execute programming associated with the button press.
Speaker 16 provides audible feedback to a user of mobile device 10. When mobile device 10 receives an incoming message, speaker 16 produces an audible notification sound to alert a user to the received message. An incoming telephone call causes a ringing sound from speaker 16 to alert the user. In other embodiments, a musical ringtone, selectable via the GUI on touchscreen 12, is played via speaker 16 when an incoming telephone call is received. When mobile device 10 is used to participate in a telephone call, a user of the mobile device speaks into microphone 17 while the other conversation participants' voices are reproduced by speaker 16. When a user watches a movie or plays a game, the sound associated with the movie or game is produced by speaker 16 for the user to hear.
Front facing camera 18 provides visual feedback to the operating system of mobile device 10. Camera 18 creates a digital image of the area facing touchscreen 12. Camera 18 is used in video chat applications running on mobile device 10 to capture a user's face during a conversation. Mobile device 10 transmits the video of a user to another mobile device in another location, and receives a streaming video of another person using the other mobile device which is displayed on touchscreen 12. Camera 18 is also used to take selfies or other pictures. When camera 18 is used to take pictures, touchscreen 12 displays the image being captured by the camera so that the touchscreen is an electronic viewfinder. Captured photographs are stored on memory within mobile device 10 for subsequent viewing on touchscreen 12, sharing on social networks, or backing up to a personal computer.
Housing 20 provides structural support and protection for the internal components of mobile device 10. Housing 20 is made of rigid plastic or metallic materials to withstand environmental hazards which cause harm to the circuit boards and other components within mobile device 10 if exposed directly. In one embodiment, a panel of housing 20 opposite touchscreen 12 is removable to expose interchangeable parts of mobile device 10 such as a subscriber identification module (SIM) card, flash memory card, or battery. Housing 20 includes a transparent glass or plastic portion over touchscreen 12, which protects the touchscreen from environmental factors while allowing a user's touch to be sensed through the housing.
FIG. 1b illustrates a user 30 operating mobile device 10 as a telephone. User 30 holds mobile device 10 with speaker 16 over an ear of the user. Microphone 17 is oriented toward a mouth of user 30. When user 30 speaks, microphone 17 detects and digitizes the user's voice for transmission to a person the user is speaking with. The person that user 30 is speaking with transmits a digitized voice signal to mobile device 10 which is reproduced on speaker 16 and heard by the user. User 30 thereby converses with another person using mobile device 10.
When user 30 holds mobile device 10 as illustrated in FIG. 1b, proximity sensor 11 notifies the operating system that the user is in proximity. The operating system of mobile device 10 executes code to reduce RF output, disable touchscreen 12, and perform any other actions as programmed.
FIG. 1c illustrates user 30 touching or pressing a button 14. Pressing buttons 14 performs various actions of the operating system depending on the programming of mobile device 10. In one embodiment, user 30 presses a home button to return the display of touchscreen 12 to a home screen.
Proximity sensor 11 and buttons 14 each operate by measuring the self-capacitance of a corresponding sensing element located within mobile device 10. Self-capacitance of a sensing element increases as an object or a body part of user 30 is moved toward the sensing element. Self-capacitance of a sensing element decreases as an object or a body part of user 30 is moved further away from the sensing element. The self-capacitance of a sensing element is compared against a threshold to determine whether user 30 is in proximity to the sensing element. In other embodiments, the self-capacitance value of a sensing element is converted into a measurement of the distance between mobile device 10 and user 30. Measuring the distance of an object from mobile device 10 in a direction perpendicular to touchscreen 12 is known as z-axis detection.
Capacitive touch sensing utilizes shielding planes under the sensing elements to provide directionality of sensing and reduce interference from noise. The shielding planes for capacitive touch sensing are driven by the integrated circuit (IC) which senses the self-capacitance of the sensing elements. A sensing IC drives shielding planes to approximately the same voltage potential as an associated sensing element when detecting proximity. For proximity and z-axis distance of user 30 to be accurately detected, a sensing IC must be able to maintain a shielding plane near the same voltage as a sensing element. When the voltage of a shielding area and a corresponding sensing element are different, the shielding area contributes to the self-capacitance of the sensing area, thus affecting the proximity reading.
One goal of mobile device manufacturers is to provide capacitive touch sensing in multiple areas of a mobile device. Using more proximity sensors located at different locations allows mobile device manufacturers to implement the advanced functionality that consumers demand. However, when proximity sensing is required at distant areas of mobile device 10, using a single proximity sensing integrated circuit becomes challenging. Proximity sensor ICs available on the market include multiple sensing element terminals, but only a single terminal to connect a shielding plane. Driving multiple shielding planes at distant locations of mobile device 10 with a single shield output reduces the ability of a sensing IC to maintain the shielding planes at a voltage close to the voltage of an individual sensing element. The shield terminal is driving a greater load than is actually necessary for measurement of an individual sensing element. Additionally, RF and other interference from one area of mobile device 10 affects sensing in other areas of the mobile device because of the connection of each shielding area to a common shield terminal of the sensing IC.
Multiple sensing ICs can be used to provide an isolated shielding plane for capacitive touch sensing in each area of mobile device 10. However, using multiple sensing ICs takes up additional space on the circuit boards of mobile device 10. As today's consumers demand smaller and thinner mobile devices, circuit board area becomes more valuable and limited. A second goal of manufacturers is to provide consumers with smaller and lighter mobile devices. Using multiple capacitive sensing ICs also increases costs to the manufacturer.
Capacitive sensing ICs on the market today require a tradeoff between goals of mobile device manufacturers. On the one hand, a manufacturer can use multiple sensing ICs to accurately detect proximity of a user at distant areas of a mobile device. Using multiple sensing ICs increases the size and cost of the mobile device. On the other hand, a mobile device manufacturer can use a single sensing IC which only provides a single shield terminal. A single sensing IC with a single shield terminal provides results with reduced accuracy as more shielding planes are used in more areas of the mobile device. A mobile device 10 with less accurate proximity sensors, or fewer proximity sensors, is not able to provide the advanced functionality of other mobile devices.